1. Field of the Invention
This invention relates to a wide angle photogrphic lens, and more particularly to a compact lens whose telephoto ratio is 0.96 and a half angle of view is above about 35.degree..
2. Description of the Prior Art
For use with so-called compact cameras, an attempt has been made for development of lenses of the type in which a retrofocus type used as a wide angle lens is inverted so as to provide a photographic lens in the form of a wide angle and of a small telephoto ratio.
Of lenses of this kind, there are many lenses composed of four groups or five groups having, as a rear unit, a negative meniscus lens with a convex surface thereof greatly curved toward an image. Among these lenses, a wide angle lens of small telephoto ratio, which is simple in construction and which consists of a front unit comprising a convex, a concave and a convex and a rear unit with a surface greatly curved toward an image, has an angle of view smaller than 32.degree., and if this wide angle lens is used, for example, for a 35 mm lens shutter camera, focal length was more or less 35 mm. This wide angle lens has a telephoto ratio of about 1, and even if an aspherical surface is introduced therein, the telephoto ratio is above 0.95. Where a 35 mm camera is used, it has been difficult to make a length from the front end of the lens to a film surface less than 30 mm.
For example, the lens disclosed in Japanese Patent Application Laid-Open No. 73014/80 has a telephoto ratio of about 1 and an angle of view of about 31.5.degree., and the lens disclosed in Japanese Patent Application Laid-Open No. 94317/81 has an angle of view of about 30.degree.. Japanese Patent Application Laid-Open No. 85410/83 disclose a compact construction but has F number of 4.5-5.6 which is dark and an angle of view of 30.7.degree.. Japanese Patent Application Laid-Open No. 59217/81 discloses one in which an aspherical surface is introduced into a post-group lens, which has an angle of view of 31.5.degree. and a telephoto ratio is above 0.95.
Japanese Patent Application Laid-Open Nos. 133711/81, 95207/81, 1117/83, 62608/83, 44016/84, etc. have been known which provide a five-group construction. Any of these patent applications have an angle of view smaller than 64.degree.. The lens disclosed in Japanese Patent Application Laid-Open No. 219509/83 has an angle of view of 39.degree. but a telephoto ratio is great, i.e., 1.1, which is not sufficient to form the lens into compactness.
Japanese Patent Application Laid-Open Nos. 75612/81, 94317/81 and 57106/83 have been known which provide a four-group construction and in which a negative meniscus lens gratly curved in the post-group is made of resin. However, an easily processed resin lens has a disadvantage that a change in refractive index due to a change in temperature is great therby posing a problem that backfocus is varied. This disadvantage becomes remarkable if a power of a negative lens is increased to reduce the overall length of lens. As an example, there is disclosed Japanese Patent Application Laid-Open No. 44016/84 in which a resin lens having no power if formed into an aspherical surface in order to eliminate the aforesaid disadvantage. However, this proposal has a limitation in the design of lens.